Scientists had determined that there was no chance of impact An asteroid some 250m (600ft) across has swept past the Earth. There was no chance of it hitting the planet, but astronomers trained telescopes and radar on the object to learn as much about it as they could

The asteroid - which carries the rather dull designation 2007 TU24 - passed by at a distance of 538,000km (334,000 miles), just outside the Moon's orbit.

Scientists who study so-called near-Earth objects say similar-sized rocks come by every few years.

The moment of closest approach for 2007 TU24 was 0833 GMT, Tuesday. The asteroid was only expected to be visible through amateur telescopes that were three inches (7.6cm) or larger.

Detailed observations of 2007 TU24 could reveal whether the asteroid is a solid object or simply a loose pile of space rubble.

Knowledge of how asteroids are put together will be key to working out how we might defend ourselves against future, more threatening rocks.

An explosive attack - so popular with Hollywood scriptwriters - may not be the most effective approach. Many scientists believe that giving a hostile object a gentle nudge over a long period of time may in fact be our best strategy.

Given the estimated number of near-Earth asteroids of this size (about 7,000 discovered and undiscovered objects, says the US space agency), an object similar to 2007 TU24 would be expected to pass this close to Earth, on average, about every five years or so.

The average interval between actual Earth impacts for an object of this size would be about 37,000 years, Nasa adds.

A little over a year-and-a-half ago, a 600m-wide (2,000ft) asteroid known as 2004 XP14 flew past the Earth at just about the Earth-Moon distance.

The asteroids' names include the year in which they were first identified.